Rain, Rain, Go Away. The New TCEQ Rules Are Here to Stay.
by Sam Ballard
On September 30, 2019, the Texas Commission on Environmental Quality (“TCEQ”) announced that, effective on that date, applicants for permits, permit amendments, permit modifications, and registrations for municipal solid waste (“MSW”) facilities must use updated precipitation data from the National Oceanic and Atmospheric Administration (the “NOAA Atlas 14”) for the parts of the applications that require estimating rainfall. This change will affect facility surface water drainage reports, flood studies, designs for leachate and contaminated water management systems, and designs for freeboard. The change may also impact eligibility for arid exemptions in some areas of the state.
In September 2018, NOAA released the NOAA Atlas 14 study, providing updated precipitation data in Texas. The NOAA Atlas 14 study provides precipitation frequency estimates for durations of 5 minutes through 60 days, at average recurrence intervals of 1 year through 1,000 years for the State of Texas. By definition, a precipitation frequency estimate is defined as the depth of precipitation at a specific location for a specific duration that has a certain probability of occurring in any given year. This information is used for infrastructure design and planning activities under federal, state, and local regulations. In addition, this information is used to help delineate flood risks, manage development in floodplains, and monitor flooding threats.
According to NOAA, due to decades of additional rainfall data and improved analytical methods, the precipitation frequency estimates in the recent study are more accurate than the previously available values, which were based on data from the 1960s and 1970s. Therefore, the updated values will now supersede the previous values.
The NOAA Atlas 14 rainfall depth values may have either increased or decreased, depending on the specific location in the state. Generally, however, the updated study found increased values in parts of Texas, including larger cities, such as Austin and Houston, which will result in changes to the rainfall amounts that define 100-year flood events (a 100-year flood event has only a one percent chance of occurring in a given year). In Austin, for example, 100-year rainfall amounts for 24 hours increased as much as three inches—up to 13 inches from 10 inches—based on the study. Likewise, 100-year estimates for the Houston area increased from 13 inches to 18 inches and values previously classified as 100-year events are now more likely to occur during 25-year events, according to the study.
In addition, TCEQ has indicated that assessment of surface water drainage (both existing and proposed) for a facility must follow the methods described in the September 2019 version of the Texas Department of Transportation (“TxDOT”) Bridge Division Hydraulic Design Manual.
On September 12, 2019, TxDOT issued an updated Hydraulic Design Manual, which refers to the NOAA Atlas 14 study for its Depth-Duration-Frequency (“DDF”) data. The manual also indicates that the NOAA Atlas 14 data has superseded all previous DDF data sources for Texas.
According to the Hydraulic Design Manual, TxDOT oversees drainage practices and design standards for the creation of hydraulic facilities associated with transportation projects, including open channels, bridges, culverts, storm drains, pump stations, and storm-water quantity and quality control systems. However, although the TxDOT hydrology standards were only intended to apply to transportation projects, TCEQ regulations require MSW facilities to determine “drainage characteristics” for proposed MSW sites by applying several formulas prescribed in the TxDOT manual.
For example, pursuant to 30 Texas Administrative Code § 330.305, MSW applicants are required to assess the existing and proposed drainage characteristics of a site under 200 acres using the “rational method” formula coefficients as specified in the TxDOT manual. Additional hydraulic calculations required for permit applications under Subchapter G of Chapter 330, concerning surface water drainage, are provided in section four of the TxDOT manual, including the calculation of surface flow rates and soil water retention.
According to Robert “Holly” Holder, P.E., Environmental Sector Director at Parkhill, Smith & Cooper, Inc., the new requirements will have some degree of impact on MSW facilities statewide—some increases and some decreases. However, the greatest impacts of storm intensities are located to the west of central Texas (including facilities located in Langtry, Del Rio, Eagle Pass, and Brackettville, Texas) and eastward to the state line with Louisiana, as well as in Houston, as those areas experienced the greatest percentage increase in 100-year 24-hour estimates based on Figure 7.4 of the NOAA Atlas 14 study.
Holder also indicated that the previous annual precipitation map was recently updated, now showing a slight bulge west along a line from San Marcos to Del Rio, Texas. Any arid-exempt MSW facilities in those particular parts of the state may be at risk of losing their exemption based on the new criteria. Also revised was the 30-year annual rainfall mean annual precipitation map (figure A.3-1 of the study) that shows not only this bulge, but also that the 25-inch per year line, which has been the dividing line for arid exempt sites, has moved west along with the water balance final cover line. Some counties may now be situated east of this line, which could impact their MSW facilities. Holder also noted that TCEQ will not be requiring drainage plans to be updated at this time, but any modification or amendment request will trigger the requirement for applicants to revisit their drainage plan and include the new NOAA Atlas 14 data.
Clearly, it is essential that MSW applicants consult with their drainage engineers about potential impacts before making any permit amendments or modifications, especially if such amendments or modifications concern drainage or leachate design.
Sam Ballard is an Associate in the Firm’s Air and Waste, and Compliance and Enforcement Practice Groups. If you have any questions or would like more information about this article or other matters, please contact Sam at 512.322.5825 or firstname.lastname@example.org.